This invention relates to an arrangement in a boom for a rock drilling unit, comprising a boom pivotally connected to a frame, relative thereto, about a first shaft, and at the other end of the boom, relative thereto, a feed beam pivotally connected about a second shaft, parallel to the first shaft, a lift cylinder between the boom and the frame for turning the boom relative to the frame, a swing cylinder between the feed beam and the boom for turning the feed beam relative to the boom, wherein the lift cylinder comprises a first and a second cylinder space where hydraulic fluid is fed for turning the boom in different directions relative to the frame, and the swing cylinder comprises respective cylinder spaces where hydraulic fluid is fed for turning the feed beam into different directions relative to the boom.
A problem with rock drilling units is to maintain the alignment of the feed beam of the rock drill when the boom between the frame or the feed beam is horizontally or vertically turned for placing the drill rod at a new hole to be drilled. For this purpose so-called parallel automatism is normally used, wherein the pivotal movement of the boom relative to the frame is compensated for in joints between the feed beam and the boom by using separate servo cylinders, whereat the turning of the boom causes the length of one servo cylinder to change, which again causes the hydraulic fluid in a servo cylinder between the boom and the feed beam to shift so that the length of that cylinder is correspondingly changed and consequently the feed beam turns into a reverse direction relative to the end of the boom as compared with the boom relative to the frame. Such solutions are disclosed e.g. in SE patent 227821.
In known solutions, separate hydraulic cylinders, interconnected to form a closed circuit, are needed to maintain parallelism. Such a structure is, however, expensive and requires extra space around joints simultaneously increasing the number of wear components. A further problem is that, because the functioning of these cylinders has to be secured by using separate pressure-controlled non-return valves which close the pressure ducts of the cylinders so that a possible breakage of a hose does not allow the feed beam to turn freely, the amount of pressure needed to open these valves is harmful to the turning function of the boom as it resists the turning until an adequate pressure level is reached. As a result, boom control may be unsteady in the extreme angular positions, and in some cases the feed beam may even have to be moved to a more suitable position of equilibrium for proper control. This again complicates the drilling work and impairs the usability of the unit.
The object of the present invention is to provide an arrangement which eliminates problems of known solutions and provides a simple, easily and reliably functioning parallel control for a feed beam. The arrangement of the invention is characterized in that the piston rod of a lift cylinder is hollow, that the lift cylinder comprises a separate fixed piston extending into the piston rod, whereat a third cylinder space inside the piston rod is completely separate from the cylinder space, that the third cylinder space of the lift cylinder is connected with a first cylinder space of a swing cylinder, and that a second cylinder space of the swing cylinder is connectable with either a hydraulic fluid supply or correspondingly a hydraulic fluid tank simultaneously with a first cylinder space of the lift cylinder, whereat, when the lift cylinder retracts, the swing cylinder retracts correspondingly, and when the lift cylinder extends, the swing cylinder extends so as to essentially maintain the alignment of the feed beam irrespective of the swing angle of the boom.
It is an essential idea of the invention that a hydraulic cylinder between a boom and a frame is used for controlling the swing angle between a feed beam and the boom, said cylinder comprising a hollow piston rod and a separate fixed servo piston inside the piston rod so that with the piston moving relative to the cylinder, the volume of the space inside the piston rod changes correspondingly, proportionally to the travel length, whereat, with the piston rod extending inside the cylinder, the hydraulic fluid in said space flows out and may be used for controlling the cylinder between the feed beam and the boom. It is a further essential idea of the invention that hydraulic fluid is fed into a second space of the swing cylinder between the feed beam and the boom simultaneously as hydraulic fluid is fed into a cylinder between the boom and the frame, so that the fluid causes the boom to rise by feeding the swing cylinder between the feed beam and the boom into a direction where the hydraulic fluid discharged from it transmits feed pressure into the space inside the piston rod so that with the cylinder extending, i.e. the boom being lifted, both the area of the fixed piston inside the piston rod and the area of the moving piston have a parallel effect.
It is an advantage of the solution of the invention that when a boom is lifted upward, a large area may be used for the lifting, as the hydraulic fluid pressure being fed has a parallel effect in both cylinder spaces. Similarly, when a cylinder is lowered, the pressure retracting the cylinder works parallel with the weight effect of the boom, but in the cylinder space inside the piston rod, the pressure has a reverse effect thus compensating for the weight effect of the boom. Thus a better control in the lifting and lowering of the boom is achieved and simultaneously the desired parallelism in a feed beam is also maintained.
The invention will be described in greater detail with reference to the accompanying drawings.